The invention relates to an arrangement of busbars for conducting direct electric current from the cathode bar ends of one transverse electrolytic reduction cell, in particular such a cell for producing aluminum, to the long side of the anode beam of the next cell, via cathode busbars, connecting busbars and risers, and such that a part of the connecting busbars is positioned under the cell.
In the fused salt electrolytic process for producing aluminum, aluminum oxide is dissolved in a fluoride melt comprised for the greater part of cryolite. The cathodically precipitated aluminum collects on the floor of the cell underneath the fluoride melt, the surface of that liquid aluminum itself acting as the cathode. Dipping into the melt from above are anodes which, in conventional processes, are made of amorphous carbon. At the carbon anodes oxygen is formed as a rsult of the decomposition of the aluminum oxide; this oxygen then combines with the carbon of the anodes to form CO.sub.2 and CO. The electrolytic process takes place in the temperature range of approx. 940.degree.-970.degree. C.
In the course of the process the electrolyte becomes depleted in aluminum oxide. When the concentration of aluminum oxide in the electrolyte reaches a lower limit of 1-2 wt%, the anode effect occurs, resulting in an increase in voltage from 4-5 V to 30 V and higher. Then at the latest the crust of solid electrolyte must be broken open and the concentration of aluminum oxide raised by adding alumina to the bath.
A smelter pot room has at least two rows of longitudinal or transverse cells through which the direct electric current flows in series. In each row of cells there is always at least one return conductor bar which produces a vertical magnetic force, which markedly disturbs the desired magnetic symmetry in the cell. These vertical components of induced magnetic field are the main cause of the magnetic effects viz., stirring and doming of the metal in the pot; the reason for this is that they interact mainly with the horizontal components of current density in the metal to produce strong magnetic forces.
The electrolyzing current which flows through the anode beam, the anode rods, the anodes, electrolyte, liquid metal, carbon floor and cathode bars produces a self-consistent magnetic field in the cell with strong vertical components in the four corners. If the busbars connecting the ends of the cathode bars of one cell to the anode beam of the next cell are arranged symmetrically, they tend to reinforce this self-consistent field.
Recently therefore various efforts have been made to lead the connecting busbars from transverse cells in such a way that the vertical components of this self-consistent field are compensated as much as possible by the magnetic field of the connecting busbars. However, attention must be given to the influence of the vertical magnetic forces from the return conductors i.e. the neighboring row of cells. Attempts to compensate for this effect have been made by arranging the connecting busbars asymmetric with respect to the transverse axis of the cell.
In the German patent application DE-OS No. 26 53 643 compensation of the vertical magnetic forces is attempted by connecting different numbers of cathode bar ends on at least one side of the transverse cell to the busbar leading to the anode beam of the next cell. In terms of an additional magnetic field, this has the same effect as separating the cathode busbar at a particular point.
In the U.S. Pat. No. 4,224,127 cells for producing aluminum by the fused salt electrolytic process are described in which the electric current leaving the cell via the cathode bar ends at the long sides of the cell is conducted asymmetrically to the anode beam of the next cell via at least four collector busbars. These collector or connecting busbars leading the current off in opposite directions are arranged at different spacings on both long sides of the cell, however such that the distances between two diametrically opposite collector busbars are the same.
In contrast to these two published items, which are aimed mainly at compensating for the vertical magnetic forces produced by the return conductors, in the U.S. Pat. No. 3,969,213 an attempt is made to compensate for the self consistent field of the cell by special arrangement of the connecting busbars. In the U.S. Pat. No. 3,969,213 there are two types of connecting busbars:
The first type takes the current from one or more upstream cathode bar end and conducts this via flexible strips under the cell, in the direction of the transverse axis, to the middle, and from there in the longitudinal direction of the cell to a common connecting busbar which is situated beyond the end wall of the cell and leads to the riser to the next cell.
The downstream cathode bar ends are connected in groups to a second kind of connecting busbar which runs along the long side of the cell to the previously mentioned common connecting busbar.
In U.S. Pat. No. 3,969,213 by displacing the symmetry with respect to the transverse axis of the cell it is possible to compensate for the vertical magnetic forces due to the return conductor bars.